Yagi antennas would not be very suitable for GSM applications unless you are trying to cover a tunnel or similar area. The problem is that Yagi's typically have very narrow and very directional beamwidths. This would cause the coverage area very specific. On the phone side, this would be even more impratical because if you are not aiming the yagi right at the transmitter (or strongest mulitpath signal) then the antenna would not be much good. This would require that you know where the transmitter or strongest multipath signal is at all times. Can you see how this would be impossible for a mobile phone user?
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